Inhibition of Axl activation diminishes FLT3 phosphorylation in FLT3-ITD⁺ AML cells. (A) MV4;11 human FLT3-ITD⁺ AML cell line (left) or primary FLT3-ITD⁺ AML blasts (right) were treated with Ctrl-Fc or Axl-Fc for the indicated times and subject to immunoblot to detect phospho-FLT3 and FLT3. Cells were treated with Ctrl-Fc for 6 hours. (B) MV4;11 human FLT3-ITD⁺ AML cell line was transfected with Ctrl or Axl siRNA and were then harvested 16 hours after transfection and subjected to immunoblot. A densitometry measurement is shown below each band. Actin was used as a loading control. (C) After the MV4;11 human FLT3-ITD⁺ AML cell line was treated with Ctrl-Fc or Axl-Fc for the indicated times, cells were then lysed and coimmunoprecipitated using anti-Axl (left) or anti-FLT3 (right) rabbit IgG. Immunoblots were performed to detect FLT3, Axl, rabbit IgG, and human IgG. Recombinant Axl-Fc protein was used as a positive control for human IgG.